Between two Outlook or Microsoft 365 users, receipts often work if neither side blocks them. From Outlook to a personal Gmail address, a receipt usually does not come back. Between Workspace accounts it depends on what the administrators allow.
A tracking image is loaded by most programs that display images, whatever the provider. But Gmail, Outlook.com and Yahoo often load it through their own image servers, and Apple Mail may load it in advance. HawkOpen marks these cases with their own rule codes, such as R-OPEN-GOOGLE, R-OPEN-MSPROXY, R-OPEN-YAHOO and R-OPEN-APPLE.